Optical filters are an essential part of today's optical networks. Multiwavelength filters can process several optical channels at the same time. Fabry-Perot interferometers and ring resonators are the most commonly used multiwavelength filters. Optical switches and electro-optical modulators based on ring resonators have been demonstrated in the literature. These filters have a constant free spectral range (FSR) because of invariable mechanical dimensions. FSR is defined as the wavelength spacing between two successive resonances in an interferometer.
Filters with tunable FSR provide more flexibility in the optical network. If needs of the network change, the filter can be tuned to new wavelengths. Future optical networks may use flexible grid where the channel spacing might change depending on the network conditions. In a data center if the traffic is high more wavelengths can be used to increase data transmission capacity. More wavelengths means filters and switches with smaller FSR are needed.